As a technology for expanding a wireless communication band, frequency aggregation using a bundle of frequencies has been known. In the use of frequency aggregation, for example, a 10-megahertz width is used for a band of 800 megahertz, a 30-megahertz width is used for a band of 2 gigahertz, and a 60-megahertz width is used for a band of 4 gigahertz, thereby ensuring a band width of 100 megahertz in total (see Japanese Laid-open Patent Publication No. 2007-306167).
When frequency aggregation is applied to a wireless communication apparatus using Orthogonal Frequency-Division Multiplexing (OFDM) as a modulation technique, however, a demodulation process at a receiving side may be complicated. This complication of the demodulation process is explained with reference to FIG. 10.
In mobile communication, the higher the frequency for use is, the more the wireless communication apparatus is susceptible to Doppler shift. When OFDM is used as a modulation technique, the influence of Doppler shift can be reduced by widening a sub-carrier interval. However, since widening a sub-carrier interval decreases efficiency in use of frequency, the sub-carrier interval is preferably as narrow as possible in the case of low frequency.
In view of such circumstances, when frequency aggregation is applied to a wireless communication apparatus using OFDM as a modulation technique, it would be preferable to change a sub-carrier interval for each frequency band. In OFDM, however, since the sub-carrier interval is in correlation with a sampling interval, if the sub-carrier interval is changed for each frequency band, the sample length differs for each frequency band.
As depicted in FIG. 10, an OFDM frame includes a plurality of OFDM symbols and cyclic prefixes (CP) each disposed before every OFDM symbol. Also, the length of each of the OFDM symbols and the length of each CP are influenced by the sample length. Therefore, if the sample length differs for each frequency band, as depicted in the example in FIG. 10, the frame length of a frame at a frequency f0 may differ from the frame length of a frame at a frequency f1.
As such, if the frame length differs for each frequency band, in frequency aggregation using a bundle of frequencies, demodulation timing is required to be detected at a receiving side for each frequency band, thereby complicating the demodulation process. Although the frame length of each frequency band can be adjusted to some degree by changing the number of OFDM symbols and the length of each CP in a frame, only such adjustment is not enough to make the frame size uniform.